INFLUENCE OF LIGHT ON THE PEOPAGATION OF SOUNP. 31 



Full as I was of the subject, I thought of means of con- An instrument 



fiimini? the reality of the phenomenon by the help of an in- '" "i*-s"-"-ethe 

 ^ •' ' •' ' . propagation of 



strument, which, placing me above all doubt both with re-sound. 



gard to the variations of the atmosphere, and as to any illu- 

 sion that might have deceived my senses, should give me a 

 just measure of the increase of the propagation of sound by 

 the influence of light. 



The construction of this apparatus offered its difficulties. 

 The principal was to subject to mechanical proof an object, 

 of which we are accustomed to judge only by our senses. 

 Whatever precision might be desired in such a research, 

 I conceived I might derive some assistance from the expe- 

 rience acquired in practising music, to attain satisfactory 

 results. 



An experienced ear, the habit of playing on musical 

 instruments, and the desire of doing right, appeared to me 

 to add to the mechanical means of my apparatus that 

 degree of accuracy, which is requisite in physical de- 

 monstrations. The following were my ideas and modes of 

 proceeding. 



In whatever manner the vibration of a body may be coru- Thevibratioi 

 municated to the elastic fluids surrounding it, it is certain, of the elastic 

 that the vibration of these fluids is always analogous to that ['"'J''na!ogou& 

 !• 1 1 to those of the 



of tne particles of the soundmg body. A chord stretched particles of the 

 and struck with the finger vibrates in a given mode an(i sound'"£ body, 

 time. Tnis different regularity in its vibrations it is, that 

 forms the duration of the sound, and the nature of the tone. 

 When a chord vibrates, the tremulation of the circumam- 

 bient air, which is analogous to the motion of the chord, 

 can communicate itself to a similar chord, if the dimen- 

 sions and tension are in corresponding proportions. It 

 is a known fact, that if two strings, belonging to two in- 

 struments, be in unison, we cannot touch one, without the 

 other's vibrating, and emitting a perceptible sound. I 

 thought i might avail myself of this property of elastic fluids, 

 to determine the mode of my experiments. 



I took two violins of good quality, fitted them up Apparatus dc 

 with well chosen Naples strings, and had the pegs madescribed. 

 with copper screws, that I might graduate the tones with 

 precision. I placed these two violinshorizontallj on a plank 



tea 



